diff --git a/src/SPC/builder/linux/library/icu.php b/src/SPC/builder/linux/library/icu.php
index 7b8bcf8e..3a41cb13 100644
--- a/src/SPC/builder/linux/library/icu.php
+++ b/src/SPC/builder/linux/library/icu.php
@@ -15,9 +15,11 @@ class icu extends LinuxLibraryBase
 
     protected function build(): void
     {
+        $userCxxFlags = trim((string) getenv('SPC_DEFAULT_CXX_FLAGS'));
+        $userLdFlags = trim((string) getenv('SPC_DEFAULT_LD_FLAGS'));
         $cppflags = 'CPPFLAGS="-DU_CHARSET_IS_UTF8=1  -DU_USING_ICU_NAMESPACE=1 -DU_STATIC_IMPLEMENTATION=1 -DPIC -fPIC"';
-        $cxxflags = 'CXXFLAGS="-std=c++17 -DPIC -fPIC -fno-ident"';
-        $ldflags = SPCTarget::isStatic() ? 'LDFLAGS="-static"' : '';
+        $cxxflags = "CXXFLAGS=\"-std=c++17 -DPIC -fPIC -fno-ident {$userCxxFlags}\"";
+        $ldflags = SPCTarget::isStatic() ? "LDFLAGS=\"-static {$userLdFlags}\"" : "LDFLAGS=\"{$userLdFlags}\"";
         shell()->cd($this->source_dir . '/source')->initializeEnv($this)
             ->exec(
                 "{$cppflags} {$cxxflags} {$ldflags} " .
diff --git a/src/SPC/builder/linux/library/openssl.php b/src/SPC/builder/linux/library/openssl.php
index bfc3936b..c4efdfdf 100644
--- a/src/SPC/builder/linux/library/openssl.php
+++ b/src/SPC/builder/linux/library/openssl.php
@@ -57,6 +57,11 @@ class openssl extends LinuxLibraryBase
         $openssl_dir ??= '/etc/ssl';
         $ex_lib = trim($ex_lib);
 
+        // OpenSSL's Configure ignores env CFLAGS for its target template; pass our flags as extra args after the target.
+        $userCFlags = trim((string) getenv('SPC_DEFAULT_C_FLAGS'));
+        $userLdFlags = trim((string) getenv('SPC_DEFAULT_LD_FLAGS'));
+        $userExtraFlags = trim($userCFlags . ' ' . $userLdFlags);
+
         shell()->cd($this->source_dir)->initializeEnv($this)
             ->exec(
                 "{$env} ./Configure no-shared {$extra} " .
@@ -67,7 +72,8 @@ class openssl extends LinuxLibraryBase
                 'enable-pie ' .
                 'no-legacy ' .
                 'no-tests ' .
-                "linux-{$arch}"
+                "linux-{$arch} " .
+                $userExtraFlags
             )
             ->exec('make clean')
             ->exec("make -j{$this->builder->concurrency} CNF_EX_LIBS=\"{$ex_lib}\"")
diff --git a/src/SPC/builder/unix/library/bzip2.php b/src/SPC/builder/unix/library/bzip2.php
index 32d87270..4ed3b46a 100644
--- a/src/SPC/builder/unix/library/bzip2.php
+++ b/src/SPC/builder/unix/library/bzip2.php
@@ -10,7 +10,8 @@ trait bzip2
 {
     public function patchBeforeBuild(): bool
     {
-        FileSystem::replaceFileStr($this->source_dir . '/Makefile', 'CFLAGS=-Wall', 'CFLAGS=-fPIC -Wall');
+        $extra = trim((string) getenv('SPC_DEFAULT_C_FLAGS'));
+        FileSystem::replaceFileStr($this->source_dir . '/Makefile', 'CFLAGS=-Wall', "CFLAGS=-Wall {$extra}");
         return true;
     }
 
diff --git a/src/SPC/command/CraftCommand.php b/src/SPC/command/CraftCommand.php
index 6c40133d..7a4765ac 100644
--- a/src/SPC/command/CraftCommand.php
+++ b/src/SPC/command/CraftCommand.php
@@ -19,6 +19,9 @@ class CraftCommand extends BuildCommand
     public function configure(): void
     {
         $this->addArgument('craft', null, 'Path to craft.yml file', WORKING_DIR . '/craft.yml');
+        $this->addOption('pgi', null, null, 'Forward --pgi to the inner build (instrumented binaries).');
+        $this->addOption('cs-pgi', null, null, 'Forward --cs-pgi to the inner build (cs-instrumented binaries).');
+        $this->addOption('pgo', null, null, 'Forward --pgo to the inner build (use collected profile data).');
     }
 
     public function handle(): int
@@ -105,6 +108,11 @@ class CraftCommand extends BuildCommand
         if ($craft['craft-options']['build']) {
             $args = [$static_extensions, "--with-libs={$libs}", "--build-shared={$shared_extensions}", ...array_map(fn ($x) => "--build-{$x}", $craft['sapi'])];
             $this->optionsToArguments($craft['build-options'], $args);
+            foreach (['pgi', 'cs-pgi', 'pgo'] as $pgoFlag) {
+                if ($this->getOption($pgoFlag)) {
+                    $args[] = "--{$pgoFlag}";
+                }
+            }
             $retcode = $this->runCommand('build', ...$args);
             if ($retcode !== 0) {
                 $this->output->writeln('<error>craft build failed</error>');