diff --git a/aclocal.m4 b/aclocal.m4
index 7e555446a1f1c8f424d83c2fb110be52adbc6b97..151cc2ea2c0d791762c0de9c2937266df1c02e73 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -1184,6 +1184,7 @@ AC_SUBST(GccLT46)
 
 dnl Check to see if the C compiler is clang or llvm-gcc
 dnl
+GccIsClang=NO
 AC_DEFUN([FP_CC_LLVM_BACKEND],
 [AC_REQUIRE([AC_PROG_CC])
 AC_MSG_CHECKING([whether C compiler is clang])
@@ -1191,6 +1192,7 @@ $CC -x c /dev/null -dM -E > conftest.txt 2>&1
 if grep "__clang__" conftest.txt >/dev/null 2>&1; then
   AC_SUBST([CC_CLANG_BACKEND], [1])
   AC_SUBST([CC_LLVM_BACKEND], [1])
+  GccIsClang=YES
   AC_MSG_RESULT([yes])
 else
   AC_MSG_RESULT([no])
@@ -1205,6 +1207,7 @@ else
     AC_MSG_RESULT([no])
   fi
 fi
+AC_SUBST(GccIsClang)
 
 rm -f conftest.txt
 ])
@@ -2049,7 +2052,16 @@ AC_DEFUN([FIND_GCC],[
     then
         $1="$CC"
     else
-        FP_ARG_WITH_PATH_GNU_PROG([$1], [$2], [$3])
+        FP_ARG_WITH_PATH_GNU_PROG_OPTIONAL([$1], [$2], [$3])
+        # From Xcode 5 on, OS X command line tools do not include gcc anymore. Use clang.
+        if test -z "$$1"
+        then
+            FP_ARG_WITH_PATH_GNU_PROG_OPTIONAL([$1], [clang], [clang])
+        fi
+        if test -z "$$1"
+        then
+            AC_MSG_ERROR([cannot find $3 nor clang in your PATH])
+        fi
     fi
     AC_SUBST($1)
 ])
diff --git a/mk/config.mk.in b/mk/config.mk.in
index 0c2003b68058eee5d071de3529b32378f0827aec..6402ac5680349dcd255dc1f57059867ecb488d9e 100644
--- a/mk/config.mk.in
+++ b/mk/config.mk.in
@@ -490,6 +490,7 @@ endif
 TargetPlatformFull    = @TargetPlatformFull@
 GccLT34         = @GccLT34@
 GccLT46         = @GccLT46@
+GccIsClang      = @GccIsClang@
 
 CC              = $(WhatGccIsCalled)
 CC_STAGE0       = @CC_STAGE0@
diff --git a/mk/validate-settings.mk b/mk/validate-settings.mk
index 9aa824d5bc6da3d9d289cb8d114aa5ac495d02f7..00ce02748e426d0a9a3f3529e144c793796f23bf 100644
--- a/mk/validate-settings.mk
+++ b/mk/validate-settings.mk
@@ -10,6 +10,8 @@ HADDOCK_DOCS    = YES
 #####################
 # Warnings
 
+ifeq "$(GccIsClang)" "NO"
+
 # Debian doesn't turn -Werror=unused-but-set-variable on by default, so
 # we turn it on explicitly for consistency with other users
 ifeq "$(GccLT46)" "NO"
@@ -18,6 +20,13 @@ SRC_CC_WARNING_OPTS += -Werror=unused-but-set-variable
 SRC_CC_WARNING_OPTS += -Wno-error=inline
 endif
 
+else
+
+# Don't warn about unknown GCC pragmas when using clang
+SRC_CC_WARNING_OPTS += -Wno-unknown-pragmas
+
+endif
+
 SRC_CC_OPTS     += $(WERROR) -Wall
 SRC_HC_OPTS     += $(WERROR) -Wall
 
diff --git a/utils/hp2ps/Axes.c b/utils/hp2ps/Axes.c
index 4c2e4f5e95350847326769f9c33ed1abbc27ba3d..adc3c45146394ca0d3d07a8500290921dfb8f2ba 100644
--- a/utils/hp2ps/Axes.c
+++ b/utils/hp2ps/Axes.c
@@ -203,14 +203,18 @@ Round(floatish y)
     int i;
 
     if (y > 10.0) {
-	for (i = 0; y > 10.0; y /= 10.0, i++) ;
+	for (i = 0; y > 10.0; y /= 10.0, i++)
+	    ;
 	y = OneTwoFive(y);
-	for ( ; i > 0; y = y * 10.0, i--) ;
+	for ( ; i > 0; y = y * 10.0, i--) 
+	    ;
 
     } else if (y < 1.0) {
-	for (i = 0; y < 1.0; y *= 10.0, i++) ;
+	for (i = 0; y < 1.0; y *= 10.0, i++) 
+	    ;
         y = OneTwoFive(y);
-        for ( ; i > 0; y = y / 10.0, i--) ;
+        for ( ; i > 0; y = y / 10.0, i--) 
+            ;
  
     } else {
 	y = OneTwoFive(y);