diff --git a/src/SPC/builder/unix/library/fastlz.php b/src/SPC/builder/unix/library/fastlz.php
index 49466475..998b457b 100644
--- a/src/SPC/builder/unix/library/fastlz.php
+++ b/src/SPC/builder/unix/library/fastlz.php
@@ -10,8 +10,9 @@ trait fastlz
 {
     protected function build(): void
     {
+        $extra = trim((string) getenv('SPC_DEFAULT_C_FLAGS'));
         shell()->cd($this->source_dir)->initializeEnv($this)
-            ->exec((getenv('CC') ?: 'cc') . ' -c -O3 -fPIC fastlz.c -o fastlz.o')
+            ->exec((getenv('CC') ?: 'cc') . " -c {$extra} fastlz.c -o fastlz.o")
             ->exec((getenv('AR') ?: 'ar') . ' rcs libfastlz.a fastlz.o');
 
         if (!copy($this->source_dir . '/fastlz.h', BUILD_INCLUDE_PATH . '/fastlz.h')) {
diff --git a/src/SPC/builder/unix/library/jbig.php b/src/SPC/builder/unix/library/jbig.php
index 5b00f0d6..27cb6141 100644
--- a/src/SPC/builder/unix/library/jbig.php
+++ b/src/SPC/builder/unix/library/jbig.php
@@ -10,7 +10,8 @@ trait jbig
 {
     public function patchBeforeBuild(): bool
     {
-        FileSystem::replaceFileStr($this->source_dir . '/Makefile', 'CFLAGS = -O2 -W -Wno-unused-result', 'CFLAGS = -O2 -W -Wno-unused-result -fPIC');
+        $extra = trim((string) getenv('SPC_DEFAULT_C_FLAGS'));
+        FileSystem::replaceFileStr($this->source_dir . '/Makefile', 'CFLAGS = -O2 -W -Wno-unused-result', "CFLAGS = -W -Wno-unused-result {$extra}");
         return true;
     }
 
diff --git a/src/SPC/builder/unix/library/qdbm.php b/src/SPC/builder/unix/library/qdbm.php
index 640e6c4d..712935bf 100644
--- a/src/SPC/builder/unix/library/qdbm.php
+++ b/src/SPC/builder/unix/library/qdbm.php
@@ -14,6 +14,8 @@ trait qdbm
     {
         $ac = UnixAutoconfExecutor::create($this)->configure();
         FileSystem::replaceFileRegex($this->source_dir . '/Makefile', '/MYLIBS = libqdbm.a.*/m', 'MYLIBS = libqdbm.a');
+        $extra = trim((string) getenv('SPC_DEFAULT_C_FLAGS'));
+        FileSystem::replaceFileRegex($this->source_dir . '/Makefile', '/^CFLAGS = .*$/m', "CFLAGS = -Wall {$extra}");
         $ac->make($this instanceof MacOSLibraryBase ? 'mac' : '');
         $this->patchPkgconfPrefix(['qdbm.pc']);
     }